The invention relates to a planar antenna, with annular slot, exhibiting a compact shape which is more especially intended to be integrated into user terminals of mobile radio telephone networks. These networks may be accessible to the public or else be private networks and possibly domestic networks.
The user terminals provided for such mobile radio networks are of ever smaller weight and bulk so as to satisfy the wishes of users who want to be able to carry them around easily on themselves or with themselves. The antennas provided for such terminals must therefore be of small size while yet offering high performance.
It is advantageous to integrate planar antennas made on supports of the printed circuit type into user terminals, since these supports exhibit a low profile. Under these conditions they are easily integrated into the analogue processing circuits required for the operation of the terminals and with which they exhibit a good degree of fit.
A known solution provides for the use of a planar antenna substrate which exhibits high permittivity making it possible to reduce the guided wavelength of the antenna and hence the size of the radiating element. This reduction in size is especially beneficial in the case where a terminal utilizes low frequencies, as is provided in respect of the terminals of existing networks and those currently under development and in particular in the case of GSM, WAP, GPRS, UMTS networks, etc.
However, the performance of small antennas made by utilizing such substrates with high permittivity may generally be regarded as insufficient on account of poor efficiency of structural origin and they are moreover relatively expensive.
The invention therefore proposes a novel planar antenna topology with annular slot making it possible to obtain an appreciable size reduction with a standard printed substrate which does not exhibit the drawbacks with regard to efficiency and cost which generally affect antennas made on a high-permittivity substrate.
The subject of the invention is therefore a planar antenna carried out by a substrate comprising an annular slot which is dimensioned to operate at a given frequency and which is fed via a feedline in a short-circuit plane of which it is situated.
According to a characteristic of the invention, the annulus formed by this slot, of annular shape, is deformed as indentations in at least one zone of the plane, where the electric field is a minimum for the given frequency and a given mode, so as to exhibit a lengthening of the slot perimeter with respect to an annular slot of corresponding circular form, without surface extension of the substrate zone wherein the slot is made.
According to a characteristic of the invention, the slot annulus is deformed as indentations, in at least one zone in which the electric field is a minimum, by a specified number of deformation elements and in particular by indentations relating to all or part of this zone.